It is known to use pressure switch assemblies actuatable in response to fluid pressure for detecting a predetermined condition within a hydraulic system. For example, U.S. Pat. No. 4,454,393 discloses an electronic warning device for signaling an error condition in a hydraulic system employing three pistons which are spatially arranged so that when predetermined spatial relations are established therebetween in response to fluid pressure, electronic signals are issued by switching elements to indicate an error condition.
It is also known to employ stroke switch assemblies for detecting a predetermined condition within a brake system. For example, U.S. Pat. No. 5,388,894 discloses a brake fluid distribution system for use in antilocking brake systems (ABS) which employs a switch in communication with low pressure accumulator for activating pumps that evacuate the brake fluid from the low pressure accumulator. The accumulator has an indicator piston which indicates the amount of brake fluid within the accumulator. When the amount of brake fluid within the accumulator reaches a preselected level, the indicator piston causes a switch piston to turn on the switch for activating pumps that evacuate the brake fluid.
Like the &gt;894 patent, U.S. Pat. No. 4,799,048 discloses an accumulator disposed in fluid supply tube system of a hydraulically operated device (e.g., automotive hydraulic anti-skid braking system) which employs a switch mechanism, e.g., a reed switch, to indicate when there is excessive fluid pressure in the accumulator.
The prior art pressure switch or stroke switch assemblies in hydraulic systems, especially hydraulic brake systems, pose a significant disadvantage because they do not indicate different positions of the brake-applying/brake-releasing piston. This drawback will be further illustrated with respect to spring apply, hydraulic release (SAHR) brake systems for vehicles. SAHR brakes, as known, generally operate in the following manner: a spring normally biases a piston which applies the brakes; normal operation of a brake release actuator causes pressurized hydraulic fluid to move the piston against spring bias to release the brakes.
If the movement of the piston to release the brakes in the SAHR brake system is partially blocked so that brakes are not fully released, the prior art pressure switch assemblies may still indicate that the brakes are fully released if the amount of hydraulic fluid pressure measured within the system exceeds a certain preset threshold. This may result in significant damage to the components of the brake system if an operator of the vehicle operates the vehicle believing that the brakes are fully released. Also, stroke switch assemblies, such as used in the &gt;048 and &gt;894 patents, are not very useful for controlling brake operations since the stroke switch assemblies only detect one position of the piston.
Therefore, what is desired is a system for a hydraulic system which will accurately, electrically indicate brake positions by detecting piston positions so that braking operations can be controlled to respond to changes in the hydraulic system.